1. Field of the Invention
This invention relates to an information reproducing method for reproducing information recorded on an optical recording medium by using a laser beam, and to an apparatus for executing this method.
2. Description of the Related Art
In an optical information recording/reproducing apparatus for reproducing information marks recorded on information tracks of an optical recording medium using a laser beam, the laser beam is condensed as small as possible on the optical recording medium by using an objective lens. The minimum diameter of the optical spot formed by this means on the optical information recording medium is defined substantially as xcex/NA by the wavelength xcex of the laser beam and the numerical aperture NA of the objective lens. In order to improve the recording density of the optical recording medium, on the other hand, the arrangement gaps (mark pitch) of the information marks in the optical spot scanning direction may be reduced. When the mark pitch becomes smaller than the spot diameter, however, the optical spot radiates simultaneously parts of other adjacent information marks when it radiates a target information mark. Therefore, signals of the adjacent information marks leak to the signal of the information mark that is to be reproduced (this leak will be hereinafter referred to as xe2x80x9cinter-symbol interferencexe2x80x9d). This interference interferes with noise components and lowers reproduction accuracy. In a system that includes a laser having a specific wavelength and an objective lens, the interference of the signals of the adjacent information marks renders a critical problem for achieving the high density.
A method that applies a wave form equalization processing to a reproducing signal and reduces the inter-symbol interference has been employed in the past as means for lowering the mark pitch. Hereinafter, this equalization processing method will be explained with reference to FIG. 7 that schematically shows the wave form equalization processing. A reproducing signal 104 is inputted to an amplitude adjustment circuit 500-1 and to a delay circuit 510-2. The amplitude adjustment circuit 500-1 multiplies the reproducing signal 104 by a predetermined multiple in accordance with the equalization coefficient signal 502-1 outputted from a coefficient generator 501. When the equalization coefficient signal 502-1 is Cl, for example, the reproducing signal 104 is multiplied by Cl by a multiplication circuit 505-1 contained in the amplitude adjustment circuit 500-1, and is outputted as a signal-after-amplitude adjustment 520-1. On the other hand, the reproducing signal 104 inputted to the delay circuit 510-2 is delayed by a predetermined delay amount and is converted to a signal-after-delay 511-2. The equalization processing comprises a plurality of processing as shown in FIG. 7, and is therefore executed serially. In consequence, signals-after-amplitude adjustment 520-1 to 520-n, each receiving an intrinsic delay amount and an intrinsic amplitude change, are acquired. These signals-after-amplitude adjustments 520-1 to 520-n are added by an addition circuit 530 and a signal-after-equalization 108 is outputted consequently. If the equalization coefficient signal 502-1 to 502-n outputted from each coefficient generator 501-1 to 501-n is set in advance to an appropriate value, the amount of the inter-symbol interference contained in the signal-after-equalization 108 can be drastically reduced. These equalization coefficients and delay amounts are set in most cases to optimum values that are determined experimentally. Incidentally, when n=3, the processing is referred to as xe2x80x9c3-tap equalization processingxe2x80x9d and when n=5, xe2x80x9c5-tap equalization processingxe2x80x9d.
Incidentally, explanation of reference numerals 500-2, 502-2, 500-n, 502-n, 510-n and 511-n will be omitted because it is the same as the explanation of the reference numerals 500-1, 502-1, 510-2 and 511-2.
The diameter of the optical spot used for reproduction is defined substantially as xcex/NA by the wavelength xcex of the laser beam and the numerical aperture NA of the objective lens, as described above. In this case, the highest temporal frequency that can be reproduced is (4xc3x97NA)/xcex. As the frequency of the highest density repetition signal recorded approaches the temporal frequency, the signal amplitude in reproduction becomes smaller, and reproduction becomes more difficult. Therefore, when the high density is achieved by reducing the mark pitch, the highest density repetition signal involves deterioration of a signal-to-noise ratio (S/N) resulting from the drop of the amplitude, and reproduction accuracy drops.
The amplitude of the highest density repetition signal can be increased generally when the inter-symbol interference is reduced by the equalization processing described above. In consequence, the S/N can be improved. However, when a higher density is attained by reducing further the mark pitch, the wave form equalization system cannot acquire a sufficient S/N improvement effect while reducing the inter-symbol interference. The result is shown in FIGS. 6A-6E. FIGS. 6A-6E show the simulation result of the reproducing signals in accordance with the Hopkins"" diffraction calculation described in xe2x80x9cJ. Opt. Soc. Am.xe2x80x9d, Vol. 69, No. 1, January (1979), pp 4-24, that executes the simulation of the optical disk reproduction process in consideration of optical diffraction due to the information marks and the numerical aperture NA of the objective lens. This simulation assumes an 8-16 modulation system using a light source wavelength of 660 nm, an objective lens numerical aperture NA of 0.6, and a recording linear density on tracks of 28 xcexcm/bit. Since a window width (Tw) is 0.14 xcexcm in this case, the highest density repetition signal is recorded as a repetition of a pattern comprising a recording mark having a length of 0.42 xcexcm and a non-recorded portion having a length of 0.42 xcexcm.
FIG. 6A shows the display of the eye pattern of the reproducing signals before processing. It can be appreciated that opening cannot be obtained sufficiently in the proximity of the slice level (level xe2x80x9c0xe2x80x9d) due to the inter-symbol interference from the preceding and subsequent recorded marks. FIG. 6B shows the eye pattern as a result of the equalization processing of this signal. The equalization processing uses 3-tap equalization processing of n=3. The coefficients are set to d1=d3=xe2x88x920.12 and d2=1.0 and the delay amount by the delay circuit is twice the window width. In consequence, the inter-symbol interference can be reduced. It can be appreciated that opening of the eye in the proximity of the slice level becomes greater than in FIG. 6A. However, the amplitude of the highest repetition signal is about ⅓ of the amplitude of the highest density repetition signal, and a sufficient S/N cannot be obtained. FIG. 6C shows the eye pattern when the coefficients are set to d1=d3=xe2x88x920.30 and d2=1.0, and the amplitude of the highest density repetition signal is increased. In this case, the edge shift becomes great, and opening in the proximity of the slice level becomes small, on the contrary, though the amplitude of the highest repetition signal becomes great. As described above, the wave form equalization processing system cannot obtain a sufficient S/N improvement effect while reducing the inter-symbol interference, and the problem encountered in achieving the high density by reducing the mark pitch remains yet unsolved.
It is an object of the present invention to provide a method capable of increasing the amplitude of the highest density repetition signal while keeping the inter-symbol interference reduced and at the same time, to provide an information reproducing apparatus for accomplishing the method of the present invention.
The present invention scans information marks recorded in tracks on a recording medium by a laser beam to generate a reproducing signal, and executes an equalization processing for reducing an inter-symbol interference by serially changing equalization coefficients in accordance with the level of the reproducing signal.
In order to have the present invention more easily understood, explanation is given first on the eye patterns.
In the eye pattern shown in FIG. 6C, the equalization processing is executed using equalization coefficients that have a large absolute value in order to reduce the inter-symbol interference generated by the continuation of short marks and short non-recorded portions, and to increase the amplitude of the highest density repetition signal. At those portions in which long marks and long non-recorded portions exist, however, the reduction of the inter-symbol interference is attempted although the inter-symbol interference does not exist there from the outset. Therefore, distortion develops in the wave form with the result that the edge shift occurs. In other words, the equalization coefficient for reducing the edge shift is different between the highest density repetition signal and other signals. For this reason, the equalization processing using the equalization coefficient that is kept fixed at a predetermined value cannot increase the amplitude of the highest density repetition signal while keeping the inter-symbol interference at a low level.
The present invention executes the equalization processing using an appropriate equalization coefficient for each signal. Hereinafter, the present system will be explained. FIG. 5 shows the relation between the absolute value of the reproducing signal and the equalization coefficient. The present system gives an appropriate equalization coefficient to each signal on the basis of this relation. The smaller the absolute value of the reproducing signal, the greater becomes the equalization coefficient. When the absolute value is 0, the equalization coefficient is a. As the absolute value of the reproducing signal becomes great, the equalization coefficient becomes small. When the absolute value of the reproducing signal attains the maximum value 1, the equalization coefficient becomes b.
When a short mark is reproduced, the absolute value of the resulting reproducing signal becomes small. When a long mark is reproduced, on the contrary, the absolute value of the resulting reproducing signal is great. In other words, according to the rule depicted in FIG. 5, a large equalization coefficient is used for a short mark. Therefore, the inter-symbol interference can be greatly reduced and the amplitude after equalization becomes greater than before. A small equalization coefficient is used for a long mark. Therefore, the amplitude after equalization does not much change than before equalization. This also holds true of the edge shift. The equalization is positively executed near the portions where the absolute value is small, that is, the portions where the inter-symbol interference is likely to occur because the mark length and the length of the non-recorded portions are small. On the other hand, the equalization is hardly executed near the portions where the absolute value is great, that is, near the portions where the inter-symbol interference is difficult to occur because the mark length and the length of the non-recorded portions are large. As a result, the waveform distortion resulting from excessive equalization can be eliminated, in principle.